In a web printing press which is a machine to print on a rolled paper, a folder, which folds printed paper fed from the main unit of the printing press (hereinafter referred to as "printing machine"), is provided with a safety device which detects paper jamming and stops operating the printing machine.
In this case, detection of paper jamming is achieved by using a limit switch or a photoelectric sensor, which is located at a position where the signature tends to cause jamming, for example, in the course of the signature flowing path, to detect staying of the signature for a time exceeding a predetermined period.
However, in such paper jamming detection by a limit switch or photoelectric sensor, there has been a problem in that, due to a high printing speed, a large amount of paper tends to build up during the time from the beginning of paper jamming to the detection of the jamming and stoppage of the printing machine by the function of the safety device, which leads to damages to the components and requires a long time for the printing machine to resume normal operation.
Then, as an alternative method to minimize paper jamming, a deviation amount is detected between an actual signature detection timing from a sensor and a timing of signature position, where the signature is to be positioned, according to the rotation phase of the folder roller to find a condition at the beginning of paper jamming.
However, in the detection of the beginning of paper jamming, the safety device tends to operate before the beginning of paper jamming to stop the machine due to a high printing speed and depending on the deviation in relative positions or on the detection accuracy. In addition, since a web printing press requires a long time from the starting to normal operation, it is often required to avoid stopping the printing machine as possible, and the detection accuracy must be adjusted not too sensitive and not too insensitive to obtain an optimum detection accuracy. However, in practice, it is difficult to satisfy both conditions.